Paleoceanography across the Pliocene–Pleistocene transition in the southern Bering Sea: dinoflagellate cysts and acritarchs from IODP Site U1341

Abstract

The latest Pliocene–earliest Pleistocene (2.828 Ma to 2.401 Ma) interval from Integrated Ocean Drilling Program Site U1341, Bowers Ridge, southern Bering Sea, northern North Pacific has been analyzed for dinoflagellate cysts, acritarchs and other palynomorphs. Based on 86 samples, with a spacing equivalent to between 500 and 6000 years, this is the most stratigraphically detailed late Cenozoic marine palynological study yet undertaken in the Bering Sea. The dinoflagellate cyst assemblages are characterized by low taxonomic richness: round brown cysts including Brigantedinium simplex, Lejeunecysta cinctoria, L. fallax, Selenopemphix nephroides, and Trinovantedinium variabile (heterotrophic taxa), and Nematosphaeropsis labyrinthus, Impagidinium cf. japonicum, and Impagidinum spp. indet. (phototrophic taxa). Marine acritarchs are represented by Cymatiosphaera? icenorum and abundant C.? invaginata. Two informal assemblage biozones and their subzones are proposed: biozone MP-A (~2.828 Ma to 2.497 Ma) and biozone MP-B (~2.477–2.401 Ma). The co-dominance of round brown cysts and the extinct high-latitude acritarch Cymatiosphaera? invaginata in biozone MP-A reflects predominantly cold and reduced-salinity surface waters with intermittent incursions of warm and higher salinity waters from the Alaskan Stream. Neither the intensification of northern hemisphere glaciation at 2.7 Ma nor intense glaciation at 2.6 Ma are clearly represented within biozone MP-A, reflecting the enclosed nature of the Bering Sea and low taxonomic richness of the marine palynomorph record. However, the MP-A/ MP-B biozone boundary at 2.47 Ma marks a major change in the hydrography of the Bering Sea, as expressed by an abrupt decline in Cymatiosphaera? invaginata and increased percentages of fresh-water algae and terrestrial plant matter, all suggesting an important increase in the influence of river discharge at Site U1341. The reduced salinity might have largely excluded Cymatiosphaera? invaginata. Biological productivity during the summer months was nonetheless sustained or even elevated judging from the relatively high concentrations of round brown cysts and invertebrate fragments throughout biozone MP-B. Sporadic incursions of the Alaska Stream continued during this time. The overall conclusions of the present study compare favourably with published interpretations based on diatoms, although the major hydrographic change at 2.47 Ma has not yet been detected in other studies of the Bering Sea.